Molybdenum is a relatively rare element that is not found in metallic form in nature. The molybdenum serves to improve hot creep properties of steel, prevent temper brittleness of steel, and increase corrosion resistance of steel, which is a very important element to manufacture heat resistant steel or to manufacture corrosion resistant steel as an alloy element.
Molybdenite (MoS2) is a primary raw material economically obtained. Generally, a relatively low concentration of about 0.05 to 0.1 wt % molybdenite (MoS2) is included in raw ore; however, the molybdenite is easily recovered and concentrated by froth flotation due to properties of sulfides. The natural resource of usable molybdenite is mainly distributed in countries such as China, USA, Chile, or the like, which is mainly produced from a by-product of a copper mine.
Generally, the copper content of ferromolybdenum for making steel is limited to 0.5% or less. In order to lower the copper content of the molybdenite, degradation in recovery rate of molybdenum is inevitable because copper ore is also sulfide form. Meanwhile, molybdenite concentrate with high copper content is also produced and sold in some mines. Therefore, in order to use the molybdenite with high copper content, the copper content is lowered by using an acid leaching process after oxidation or by being mixed with ores with low copper content.
The ferromolybdenum implies an alloy of 50 to 75 wt % molybdenum and remaining iron, which is mainly used to add molybdenum during a steelmaking process. Generally, the ferromolybdenum is manufactured by a metallothermic reduction (Thermit) method that mixes molybdenum oxide (MoO3) and iron oxide with a strong reducing agent, i.e., aluminum, and then reacts them. The metallothermic reduction method instantly generates a large amount of heat while oxidizing the aluminum by depriving oxygen from the molybdenum oxide or the iron oxide, such that the reaction temperature reaches a high temperature of 3000° C. or higher. In this case, when copper is included in a raw material, the copper is also reduced and thus, most of the copper exists in the metal, i.e, the ferromolybdenum alloy layer rather than in the oxide slag. Therefore, the copper content of the molybdenum oxide that is a raw material is strictly restricted.
Most of the molybdenum oxide is manufactured by roasting the molybdenite in the air at 560 to 600° C. When the copper content of the molybdenite is high, the copper is removed by acid-leaching oxidized ores after roasting and filtering it. During this process, since a considerable amount of molybdenum is eluted and thus, exists in the extracting solution, it is recovered by solvent extraction or pH control. During the roasting, a large amount of heat is generated by the combustion of molybdenum and sulfur. That is, the oxidation state of the molybdenum in the molybdenite is +4 and the oxidation state thereof in the oxidized ores is +6. Therefore, a larger amount of reducing agent than the molybdenite is needed in order to manufacture the ferromolybdenum from the oxidized ores. In addition, the metallothermic reduction process occurs explosively and completes almost immediately, such that it is difficult to control the reaction and it is impossible to obtain homogeneous products.